1. Field of the Invention
The present invention relates to a technology for controlling a positional relation between components of a fixing device for use in an image forming apparatus.
2. Description of the Related Art
As well known in the art, in an image forming apparatus such as a printer, a copying machine, a facsimile machine, and a printing machine, a latent image formed on a latent image carrier is developed with toner into a toner image and the toner image is transferred and fixed onto a recording medium. Thus, a copy of an original is obtained.
As an example of a device used for fixing process, a device that adapts a heat roller fixing system is well known in the art. More specifically, in the heat roller fixing system, a toner image can be fixed due to fusing and penetrating of toner caused by heat and pressure applied to the toner image by a pair of opposing rollers that sandwich a conveying path of a recording paper.
The heat roller fixing system is generally provided with a fixing roller that is made into contact with toner and a pressure roller that is pressed toward the fixing roller. The rollers are configured such that the level of pressure to be applied to the pressure roller can be adjusted so that appropriate heat is provided to the toner. Further, the pressure roller may be configured to have a function for preventing fused toner on a recording paper passing through a fixing nip from sticking to the fixing roller because of its adhesive force while the recording paper is wound around the fixing roller. For example, Japanese Patent Application Laid-open No. 2006-48025 discloses a technology for so-called “self-stripping” of a recording paper from a fixing roller. Specifically, pressure is applied to the pressure roller to purposely deform the recording paper passing through the nip portion, so that the recording paper is bent back when the recording paper passes through the nip portion, by a shape restoring force due to a bending rigidity of the recording paper. Thus, the recording paper can be peeled off from the fixing roller.
The rollers are configured such that they always come into contact with each other but can be separated if necessary (for example, when a jammed paper sheet is removed).
For example, Japanese Patent No. 3812594 discloses a technology for controlling contacting and separating of rollers in a fixing device. Specifically, the fixing device includes a pressing lever that presses one of fixing members against the other fixing member, a pressure releasing lever rotatably supported on the pressing lever, a locking member of which one end is rotatably attached to the pressure releasing lever, and an elastic member that is locked to the other end of the locking member and pulls the locking member in a locking direction. With this configuration, a positional relation of the fixing members is controlled between a pressing position in which one of the fixing members is pressed against the other fixing member by the pressing lever and a pressure releasing position in which one of the fixing members is separated from the other fixing member, by moving a position of the locking member by rotating the pressure releasing lever.
The fixing roller is configured such that rotatably-connected pressing levers are disposed on each of the longitudinal ends of the pressure roller, or a pressing member, as well as the pressure roller is pressed against the fixing roller by biasing one end of each pressing lever by a pressing spring. The pressing levers are pressed against the fixing roller at high forces by the pressing spring to obtain the action of the self-stripping. This configuration may cause such problems that the pressing levers may be tilted or deformed, or one of the pressing levers may touch the pressing spring thereby reducing a pressing force of the fixing roller. Because of the problems, balance between the pressures applied on the left and the right ends of the pressing lever may be lost or the recording sheet may stick to the fixing roller, which supplies heat, by an adhesive force of the fused toner. Further, the toner image may be poorly fixed on the recording sheet or a layer configuration of the endless belt may be disturbed. As a result, meandering or deviated running of the endless belt occurs, resulting in damaging the endless belt or reducing the duration of life of the endless belt.